1. Field of the Invention
This invention relates to new and useful improvements in drill bits and more particularly to drill bits having diamond cutting elements and means for reinforcing the cutting elements against shear forces.
2. Brief Description of the Prior Art
Rotary drill bits used in earth drilling are primarily of two major types. One major type of drill bit is the roller cone bit having three legs depending from a bit body which support three roller cones carrying tungsten carbide teeth for cutting rock and other earth formations. Another major type of rotary drill bit is the diamond bit which has fixed teeth of industrial diamonds supported on the drill body or on metallic or carbide studs or slugs anchored in the drill body.
There are several types of diamond bits known to the drilling industry. In one type, the diamonds are a very small size and randomly distributed in a supporting matrix. Another type contains diamonds of a larger size positioned on the surface of a drill shank in a predetermined pattern. Still another type involves the use of a cutter formed of a polycrystalline diamond supported on a sintered carbide support.
Some of the most recent publications dealing with diamond bits of advanced design, relevant to this invention, consists of Rowley et al, U.S. Pat. No. 4,073,354 and Rhode et al U.S. Pat. No. 4,098,363. An example of cutting inserts using polycrystalline diamond cutters and an illustration of a drill bit using such cutters, is found in Daniels et al U.S. Pat. No. 4,156,329.
The most comprehensive treatment of this subject in the literature is probably the chapter entitled STRATAPAX BITS, pages 541-591 in ADVANCED DRILLING TECHNIQUES, by William C. Maurer, The Petroleum Publishing Company, 1421 South Sheridan Road, P.O. Box 1260, Tulsa, Okla. 74101, published in 1980. This reference illustrates and discusses in detail the development of the STRATAPAX diamond cutting elements by General Electric and gives several examples of commercial drill bits and prototypes using such cutting elements.
As is evident from a prior art cutting element 100 depicted in FIG. 12, it is conventional to provide a stud 101 with an angularly oriented supporting surface 102 to which a carbide layer of a disc-shaped element 104 is bonded. A problem which must be dealt with in such an arrangement is a tendency for the disc to be sheared from the supporting surface in response to the forces applied to the disc which can be resolved into a resultant force 106.
In an effort to deal with the shearing-off problem, it has been proposed, as depicted in FIG. 13, to form the supporting surface with a shoulder 110 located just behind, and in contact with, the disc to support the latter against shear forces. However, such a configuration creates a new problem. That is, any change in direction of the surface creates a weakened region which is susceptible to fracture. Thus, the notch 112 at the junction of the surface segments 108, 110 defines a weaker region which renders the stud susceptible to being fractured along a line 114 which propogates from the notch.